In a wire-cut EDM machine which performs submerged machining, various methods have been proposed for sealing the lower arm unit with the processing tank. For example, as disclosed in U.S. Pat. No. 4,918,279, a slide seal may seal the arm unit when the arm unit moves axially. In such machines, however, particles produced during machining of the workpiece disperse throughout the working fluid and may become deposited onto the arm unit. As a result of these particles, the slide seal may become worn and the sliding resistance of the arm unit varies significantly.
In Japanese Patent Publication 63-306827, an EDM machine is disclosed which has bellows directly connected to the arm unit to seal against axial movement of the arm unit. During machining of a workpiece, the bellows are exposed to the hydraulic pressure of the working fluid. This hydraulic pressure is also applied to the lower arm unit which may cause the lower arm unit to deform.
U.S. Pat. No. 4,992,640 issued to Tanaka, et al. also discloses the use of bellows. These bellows are connected at one end to a shielding plate and at the other end to the column. The lower arm unit is directly exposed to the hydraulic pressure inside the processing tank.
Additionally, such EDM machines may be prone to errors caused from varying temperatures. The temperature inside the processing tank is maintained at a fixed level by controlling the temperature of the working fluid. Portions of the lower arm unit outside of the processing tank, however, would be exposed to the external temperature. Because of the temperature differential, the lower arm unit may expand and cause an error of a few micrometers in the direction. Since the arm's length is about 300 mm-600 mm, a change of 1 degree C. would cause a significant thermal displacement in the axial direction.